Conventionally, to enhance the durability of a valve seat for an engine cylinder head, a wear resisting clad layer has been formed as a valve seat on the contact surface with which an intake valve or an exhaust valve comes into contact. The clad layer is often formed by melting a wear resisting material (material for cladding) by using a high-density heat source such as a laser and by depositing the melted material on the contact surface.
In the case where a cast iron material is used for a cylinder head body, since the cast iron material has a higher carbon content than the general steel material, and in particular, A-type graphite that is long and big in a flat shape is present on the surface thereof, the graphite reacts with oxygen contained in the atmosphere at the time of cladding, so that carbon dioxide gas may be yielded. By this gas yielding, gas defects such as pinholes or blowholes may be produced in the formed valve seat (clad layer).
On the other hand, since the melted material is deposited at the time of cladding, the surface of the cast iron material is heated, and the surface layer including the surface may be chilled. By this chilling, the surface layer of cast iron material is solidification-shrunk, so that bead cracks may be generated in the clad layer.
To solve the above problems, as one example of a method for manufacturing a cast iron member formed with a clad layer on the surface of cast iron material, a method has been proposed in which the surface of cast iron material to be formed with the clad layer is blasted before being clad to remove, in advance, the graphite that is present in the surface part of cast iron material, and the clad layer is formed in the surface part from which the graphite has been removed (for example, refer to Patent Document 1). According to this cladding method, since the graphite has been removed in advance from the surface part of cast iron material, the yielding of carbon dioxide gas at the time of cladding is restrained, so that gas defects in the clad layer can be restrained from being produced.
On the other hand, as one example of a method for preventing the bead cracks, a method has been proposed in which a metal (for example, cast iron) having a greater tendency to form carbide than Fe and an alloy of Ni and/or Co are arranged on the surface of cast iron, and these materials are melted by a laser and solidified to perform cladding (for example, refer to Patent Document 2). Also, a method has been proposed in which after a zone subjected most to friction of the inner surface of a cast iron-made cylinder liner has been preheated, the zone is irradiated with laser beams having high energy density, by which a material having excellent wear resistance and seizure resistance is deposited to form a clad layer (for example, refer to Patent Document 3).    Patent Document 1: JP Patent Publication (Kokai) No. 1-111855    Patent Document 2: JP Patent Publication (Kokai) No. 1-104906    Patent Document 3: JP Patent Publication (Kokai) No. 1-104487